The present invention relates to optical fiber sensors for detecting liquids.
Various optical fiber water sensors have been proposed in the prior art. Sawano et al. "Optical fiber cable with submersion sensor fiber", International Wire & Cable Symposium Proceedings 1987, p. 284, discloses an optical fiber water sensor whereby a water swellable material is disposed in a cavity adjacent a piston which confronts an optical fiber which in turn confronts a concave mandrel. Upon being exposed to water, the water swellable material swells, which urges the piston against the fiber and causes the fiber to assume a shape of the concave mandrel. This sensor is disadvantages since it is relatively complicated in design and hence expensive.
Sawano et al. further describe an optical fiber sensor whereby an optical fiber is helically wrapped about a rod composed of a water swellable material, with a stiff yarn being counter-helically wound around the rod. Upon coming in contact with water, the water swellable material swells and causes the optical fiber to bend at a plurality of points about the stiff counter-helically wound yarn. A yarn sensor merely produces a loss over a length of the yarn, rather than at a discrete point. Hence a loss trace of an OTDR looks like a high attenuation fiber instead of a step loss such as a splice and produces poor resolution. In addition, this sensor again is relatively complicated in design, takes up a relatively large amount of space, and hence is inconvenient to deploy.